A rack unit (abbreviated U or RU) is a unit of measure defined as 1 3⁄4 inches (or 44.45 mm). It’s the unit of measurement for the height of 19-inch and 23-inch rack frames and the equipment’s height. The height of the frame/equipment is expressed as multiples of rack units. A typical full-size rack is 42U high. Racks and cabinets are then listed as 1U, 2U, 3U, 4U, and so on.